The FBI is investigating whether someone intentionally punched small holes in the bellies of two US Airways (search) airliners.

No passengers were aboard the planes, which were temporarily grounded for repairs Monday at Charlotte/Douglas International Airport (search). Airline officials described the holes as similar to those made by a screwdriver, characterizing them as minor exterior damage.

Investigators said the punctures were discovered during routine inspections and resembled damage recently found on another US Airways jet in Florida. They said the punctures were unlikely to be caused by normal wear and tear.

"Anything that has the potential of damaging the structural integrity of an airplane, we're concerned about," said Kevin Kendrick, FBI special agent in charge for North Carolina.

The FBI (search) is investigating anyone who had access to the planes, including baggage and garbage handlers, mechanics, pilots and people providing the plane with fuel and food.

The two planes, a Boeing 737 (search) and an Airbus 321 (search), had come from Pensacola, Fla., and Pittsburgh.

Earlier this week, the FBI said it was investigating mysterious holes found on a plane that had flown to Orlando, Fla., after stopping in Charlotte.

The FBI has not linked the separate incidents, Kendrick said.

L. Nick Lacey, who headed flight standards for the Federal Aviation Administration (search) from 1998 to 2001, said holes on a plane's belly would be enough to delay a departure but likely not enough to pose a danger to passengers. The underside of the jets is mostly cargo space, not sensitive equipment, he said.

"I think it was interesting that it was in a visible place," he said. "If it was sabotage, there was no attempt to hide it."